One method of the prior art for making this adjustment of the illumination power is to proceed by servocontrolling this power from the brightness of the images acquired. Thus the illumination power is adjusted firstly to an arbitrary value and an image is required. The brightness of the image is measured and the illumination power is corrected depending on whether this brightness is low or on the contrary too great. A new image is acquired with this new lighting power, its brightness is measured and the power is corrected if necessary, until convergence.
This method is effective but, because it is iterative and thus requires several image acquisitions, it is expensive in terms of time and is therefore scarcely usable for acquisition on the fly.
Another method could also use the exposure measurement devices of cameras but this would require expensive instrumentation around the camera. Moreover, exposure measuring devices may be disturbed by the type of scene to be treated, for example in the case of scenes where the background is completely saturated, or by the movement of the fingers, the region of the image to be exposed correctly then being changeable.
None of these lighting control methods is suited to the acquisition of images of finger veins on the fly.